Formating Youtube preview picture?

Good morning!I have a new project here: http://www.afripix.de/solteq/On the start-screen I have a background-video with a sunset. The loop is at the moment not perfect, but that is not my question.While loading the video youtube shows a preview pic. After loading the video jumps a little bit down, it seems to be that the pic has not the same format like the video.How can I change this?